The present invention relates to a coated hard metal, particularly one that is most suitable for cutting tools, that has high resistance against wear, chipping, welding, and flaking, and that can maintain the superior properties over an extended period of time.
Tools made of coated hard metal have become commercially practical and have come into wide use. The tool has a hard metal surface which is coated to improve the cutting property. The coating is composed of one or more layers of titanium carbide, titanium nitride, titanium carbonitride, or aluminum oxide, for example. The coating is deposited by chemical vapor deposition or physical vapor deposition.
Being non-oxide layers, such titanium-based coatings are useful in improving resistance to wear and to chipping. Oxide layers such as an aluminum oxide layer and a zirconium oxide layer are considered to be suitable for use in a temperature range exceeding 700xc2x0 C. (high-speed cutting range) at the corner of a cutting tool, because they are excellent in chemical stability and resistance to heat.
Cutting work has improved, in recent years, in terms of speed and efficiency prompted by advancements in machine tools and in response to manufacturing-cost reduction demands and productivity improvement requirements. From the viewpoint of environmental protection, dry machining has been in urgent demand in order to reduce the consumption of cutting oil. In response to such a movement in the market, the use of the corner portion of a cutting tool at high-temperature range has been increasing.
In order to meet the market requirement and to lengthen the lifetime of a tool, an oxide layer excellent in chemical stability at high temperatures and in resistance to heat, such as an aluminum oxide layer and a zirconium oxide layer, has been used to coat the surface of a cutting tool, and the thickness of the oxide layer has been increased.
However, when the thickness of the oxide layer (especially the aluminum oxide layer) is increased to 1.5 xcexcm or more, the crystal grains of the oxide layer constituting the coating become coarsened, developing an unevenness on the surface of the tool in response to the grain size. This unevenness allows the chips of the work material to apply local stresses to the surface of the tool, accelerating the wear and decreasing the toughness. In addition, the chip welds itself to the uneven portion. The welded portion in turn becomes a starting point of stress application, causing flaking of the layer or chipping. As a result, the lifetime of the cutting tool is shortened.
In order to resolve this problem, published Japanese patent application Tokukouhei 5-49750 offers a method for preventing the coarsening of the crystal grains by dividing the aluminum oxide layer into multiple layers. This method undoubtedly decreases the grain size of the aluminum oxide. On the other hand, this method increases the number of interfaces between an aluminum oxide layer and an layer made of another substance, causing flaking to occur easily at the interface. The flaking develops rapid damage, decreasing the lifetime of the tool.
Another published Japanese patent application, Tokukouhei 5-57507, lengthens the lifetime of a tool by removing the unevenness of the face of the oxide layer only at the cutting edge of the tool by polishing. Although this method can increase the lifetime of the cutting edge, the aforementioned face unevenness of the oxide layer remains as a concavity on the cutting face that cannot be processed by polishing. This remaining face unevenness accelerates crater wear and other types of wear, decreasing the lifetime of the tool.
Yet another published Japanese patent application, Tokukouhei 11-124672, uses an xcex1-type aluminum oxide for an outer layer as a high-strength layer and specifies the oriented texture. This method, however, produces on the surface of the tool an unevenness corresponding to the size of the crystal grains of the aluminum oxide in the outer layer, causing a decrease in the lifetime of the tool.
Yet another published Japanese patent application, Tokukouhei 8-158052, discloses a similar layer structure to that of the present invention. However, in Tokukouhei 8-158052, the face roughness of an intermediate layer is transferred to an outer layer without improvement. The present inventors have continued to study a further increase in the lifetime of a tool, especially in the case of high-speed cutting and dry cutting, and completed the present invention.
In view of the foregoing circumstances, the main object of the present invention is to offer a coated hard-metal cutting tool that has an outer layer in which the face unevenness specific to an oxide layer is reduced, that has improved resistance to wear, to chipping, to welding, and to flaking throughout the tool, and that can maintain the superior properties for a prolonged period of time.
The present invention offers a coated hard metal having a coating on its surface. The coating comprises an inner layer, intermediate layer, and outer layer in that order from the hard-metal side. The individual layers have the following constitutions and the specified face roughness.
The inner layer includes a layer comprising at least one member selected from the group consisting of (a) the carbides, nitrides, borides, and oxides of the elements belonging to the IVa, Va, and VIa groups in the periodic table and (b) the solid solutions of these.
The intermediate layer includes a layer comprising at least one member selected from the group consisting of aluminum oxide, zirconium oxide, and their solid solution.
The outer layer includes a layer of titanium carbonitride having a columnar structure and a layer comprising at least one member selected from the group consisting of (a) the carbides, nitrides, borides, and oxides of the elements belonging to the IVa, Va, and VIa groups in the periodic table, (b) the solid solutions of these, and (c) aluminum oxide.
As for the face roughness obtained in a cross section showing the structure of the coated hard metal, the relation between xe2x80x9cAmax,xe2x80x9d which signifies the maximum roughness at the outer face (the interface) of the intermediate layer, and xe2x80x9cBmax,xe2x80x9d which signifies the maximum roughness at the outer face (the interface) of the layer of titanium carbonitride having a columnar structure in the outer layer, satisfies equation 1. It is more desirable that the relation satisfy equation 2.
(Bmax/Amax) less than 1 xe2x80x83xe2x80x83equation 1,
where 0.5 xcexcm less than Amax less than 4.5 xcexcm, and 0.5 xcexcmxe2x89xa6Bmax 4.5 xcexcm.
(Bmax/Amax) less than 0.8xe2x80x83xe2x80x83equation 2.
The face roughness is measured by the following method: A cross section perpendicular to the flank of the tool is mirror-polished. A photograph such as shown in FIG. 1 is taken under an optical microscope at 1,500 power. The unit evaluation length in a measuring area is 0.02 mm. The difference between the maximum peak height and the minimum peak height in the unit evaluation length is defined as the maximum roughness in the present invention. The maximum roughness at the outer face of the intermediate layer and the maximum roughness at the outer face of the layer of TiCN having a columnar structure in the outer layer are measured in five different fields of vision to obtain their respective average values. The average value is used as the value of the maximum roughness to calculate equation 1. Since the conventional probe method is unable to measure the face roughness at an interface, the above-described method is employed in the present invention.
When titanium carbonitride, which has high resistance to wear, is used as the outer layer, the temperature of the entire tool becomes high when the tool is used for cutting at a high-temperature range as in high-speed cutting, because titanium carbonitride has high thermal conductivity. As a result, the hard metal as the base material deforms plastically, raising the cutting resistance and thereby leading to fracture. In order to solve this problem, it is possible to provide a thick intermediate layer made of aluminum oxide or zirconium oxide, both of which have low thermal conductivity, between the hard metal and the titanium carbonitride. However, the foregoing oxide and titanium carbonitride have low strength for mutual bonding. Consequently, if an ordinary coating method is used, it will flake off during use. A thick intermediate layer has a large magnitude in face roughness and this large magnitude tends to be transferred to the outer layer, causing the coated hard metal to have an outer layer with large face roughness.
The present inventors studied a means to solve this problem and finally completed the present invention. In the present invention, in order to increase the strength of bonding between the intermediate layer and outer layer, the face roughness of the intermediate layer is increased. The area for bonding with the outer layer therefore is increased and the anchor effect is enhanced. In addition, in order to reduce the face roughness of the outer layer, titanium carbonitride having a columnar structure with a specific crystal orientation is used in the outer layer.
The present inventors found that when the aluminum oxide in the intermediate layer is cooled after the formation, cracks occur in the intermediate layer in a direction perpendicular to the layer. The cracks reduce the tensile stress in the intermediate layer and allow TiN, for example, in the outer layer to penetrate into the cracks, so a strong anchor effect can be obtained.
As can be seen from FIG. 1, the maximum roughness is large at the interface between the aluminum oxide in the intermediate layer and the titanium carbonitride in the outer layer. On the other hand, the maximum roughness is small at the surface of the outer layer. This phenomenon is expressed in equations 1 and 2. The titanium carbonitride layer having such a specific property cannot be obtained by ordinary methods. This can be achieved only when the use of an organic compound having a CN base accelerates the growing rate of crystals, producing a specific crystal orientation.
As for the inner layer, various compounds can be used that have a higher bonding strength with the oxide constituting the intermediate layer than the hard metal has. This practice is not uncommon.
The above statement can be summarized as follows:
(a) First, the surface of a hard metal is coated with an inner layer. Second, the inner layer is coated with an intermediate layer composed of an oxide layer that is excellent in chemical stability and resistance to heat and that is coarse in grain size. Third, the intermediate layer is coated with an outer layer including a layer of titanium carbonitride having a columnar structure (a crystal structure having a columnar construction) in such a manner that the face unevenness of the intermediate layer can be absorbed by the outer layer. FIG. 2 shows the columnar structure in the outer layer. This photograph shows a fractured section of a coated hard metal.
(b) Face unevenness is intentionally formed on the surface of the oxide layer in the intermediate layer so that the bonding strength between the aluminum oxide in the intermediate layer and the outer layer is increased. The conventional method was unable to provide high bonding strength between the two layers.
(c) A reduction in the face unevenness of the outer layer can improve the resistance to wear, to chipping, and to welding of the entire layer, maintaining the excellent properties for a long period of time.
The individual layers have the following limitations in thickness: It is desirable that the inner layer have a thickness of 0.1 to 10 xcexcm. If more than 10 xcexcm, the strength decreases. If less than 0.1 xcexcm, no effect can be expected. It is desirable that the intermediate layer have a thickness of 1.5 to 20 xcexcm. If less than 1.5 xcexcm, the thermal conduction cannot be suppressed. If more than 20 xcexcm, the strength tends to decrease, shortening the lifetime of the tool. It is more desirable that the intermediate layer have a thickness of 5 to 15 xcexcm. It is desirable that the outer layer have a thickness of 2 to 30 xcexcm. If less than 2 xcexcm, the face unevenness of the intermediate layer cannot be decreased effectively. If more than 30 xcexcm, the face unevenness of the outer layer itself becomes larger than that of the intermediate layer. It is more desirable that the outer layer have a thickness of 5 to 20 xcexcm.
In order to decrease the face unevenness of the intermediate layer, it is desirable that the layer of titanium carbonitride having a columnar structure in the outer layer have the largest value in the oriented texture coefficient TC at any one of the (220), (311), (331), and (422) planes and that the largest value be not less than 1.3 and not more than 3.5. It is more desirable that the TCs at the (311) and (422) planes be concurrently not less than 1.3 and not more than 3.5. In the above statement, the oriented texture coefficient TC is defined in equation 3 below.                               TC          ⁢                      xe2x80x83                    ⁢                      (            hkl            )                          =                                            I              ⁢                              xe2x80x83                            ⁢                              (                hkl                )                                                    Io              ⁢                              xe2x80x83                            ⁢                              (                hkl                )                                              ⁢                                    {                                                1                  8                                ⁢                                  xe2x80x83                                ⁢                                                      ∑                                          x                      ,                      y                      ,                      z                                                              xe2x80x83                                                        ⁢                                      xe2x80x83                                    ⁢                                                            I                      ⁢                                              xe2x80x83                                            ⁢                                              (                                                                              h                            x                                                    ⁢                                                      xe2x80x83                                                    ⁢                                                      k                            y                                                    ⁢                                                      l                            z                                                                                                                                      Io                      ⁢                                              xe2x80x83                                            ⁢                                              (                                                                              h                            x                                                    ⁢                                                      xe2x80x83                                                    ⁢                                                      k                            y                                                    ⁢                                                      xe2x80x83                                                    ⁢                                                      l                            z                                                                          )                                                                                                        }                                      -              1                                                          equation        ⁢                  xe2x80x83                ⁢        3            
where I(hkl) and I(hxkylz): the measured diffraction intensity at the (hkl) and (hxkylz) planes, respectively,
I0(hkl) and I0(hxkylz): the average value of the powder diffraction intensities of TiC and TiN at the (hkl) and (hxkylz) planes, respectively, in accordance with the ASTM Standard, and
(hkl) and (hxkylz): eight planes of (111), (200), (220), (311), (331), (420), (422), and (511), respectively.
The oriented texture coefficient TC of the layer of titanium carbonitride having a columnar structure is obtained from the diffraction peak of X-ray diffraction. In this case, the diffraction peak of the (311) plane of TiCN overlaps that of the (111) plane of WC in the base material. Consequently, the data is corrected by the following method: It is known that the peak intensity of the (111) plane of WC is 25% of that of the (101) plane of WC, the highest peak in WC. (The peak of the (101) plane of WC is shown as (A) in FIG. 3.) Accordingly, the true peak intensity of the (311) plane of TiCN is obtained by subtracting the value derived from a multiplication of the peak intensity of the (101) plane of WC by 0.25 (the derived value is equal to the peak intensity of the (111) plane of WC) from the apparent peak intensity of the (311) plane of TiCN. FIG. 3 shows a chart of the diffraction peaks obtained in Tool No. 10 in the examples.
When the oriented texture coefficient of the layer of titanium carbonitride having a columnar structure is specified to fall within the foregoing limits, the face roughness of the oxide layer in the intermediate layer can be decreased. As a result, a coated hard metal satisfying the relation expressed in equation 1 can easily be obtained, improving resistance to wear, to chipping, and to welding and performing with excellent properties.
The titanium carbonitride layer can be formed, for example, by using TiCl4, organic carbonitride, hydrogen gas, and nitrogen gas for a process at a temperature of 700 to 1,000xc2x0 C. and a pressure of not more than 667 hPa. In particular, when CH3CN is used as a source of carbon and nitrogen, producing columnar crystal grains of the TiCN is facilitated. In this case, a desirable process is as follows: The forming process is divided into two processes: the first half and second half. In the first half, the ratio xe2x80x9c(TiCl4+CH3CN)/total-gas-volumexe2x80x9d is controlled to be smaller than that of the second half, and the ratio xe2x80x9cN2/total-gas-volumexe2x80x9d is controlled to be two or more times that of the second half. It is desirable that the titanium carbonitride layer have a thickness less than 10 xcexcm.
It is desirable that the intermediate layer be composed practically of xcex1-type aluminum oxide in order to increase the mechanical strength. It is also desirable that the xcex1-type aluminum oxide have an oriented texture coefficient TCa(012) more than 1.3. In this case, the oriented texture coefficient TCa is defined in equation 4 below.                               TCa          ⁢                      xe2x80x83                    ⁢                      (            hkl            )                          =                              {                                          1                6                            ⁢                              xe2x80x83                            ⁢                                                ∑                                      x                    ,                    y                    ,                    z                                                        xe2x80x83                                                  ⁢                                  xe2x80x83                                ⁢                                                      I                    ⁢                                          xe2x80x83                                        ⁢                                          (                                                                        k                          x                                                ⁢                                                  xe2x80x83                                                ⁢                                                  k                          y                                                ⁢                                                  l                          z                                                                    )                                                                            Io                    ⁢                                          xe2x80x83                                        ⁢                                          (                                                                        h                          x                                                ⁢                                                  xe2x80x83                                                ⁢                                                  k                          y                                                ⁢                                                  xe2x80x83                                                ⁢                                                  l                          z                                                                    )                                                                                            }                                -            1                                              equation        ⁢                  xe2x80x83                ⁢        4            
where I(hkl) and I(hxkylz): the measured diffraction intensity at the (hkl) and (hxkylz) planes, respectively,
I0(hkl) and I0(hxkylz): the powder diffraction intensity of the xcex1-type crystal-structure alumna at the (hkl) and (hxkylz) planes, respectively, in accordance with the ASTM Standard, and
(hkl) and (hxkylz): six planes of (012), (104), (110), (113), (024), and (116), respectively.
In FIG. 3, diffraction peaks of aluminum oxide are hard to recognize. However, when the outer layer is removed, they appear clearly.
It is also desirable that TCa(104) and TCa(116) concurrently exceed 1.3. Such a specification of the oriented texture coefficient can produce the following effects, improving the resistance to flaking of the tool:
(a) Improvement in the mechanical strength of the oxide layer in the intermediate layer;
(b) Acceleration of the coarsening of the crystal grains of the intermediate layer; and
(c) Improvement in the strength of the mechanical bonding between the intermediate layer and the outer layer.
These effects enable the tool to perform the excellent properties of chemical stability at high temperatures and the resistance to heat, thereby extending the lifetime of the tool.
The control of the oriented texture coefficient of the aluminum oxide, for example, can be carried out in the following manner: First, individual layers up to the layer immediately below the aluminum oxide are formed. Second, the coating is exposed to a CO2 atmosphere at a pressure of 0.4 to 0.8 hPa to slightly oxidize part of the surface of the coating. Third, the aluminum oxide layer is formed at 1,000 to 1,200xc2x0 C., desirably at 1,050 to 1,150xc2x0 C., on the surface of the coating. Through this process, an xcex1-type aluminum oxide layer is formed without regard to the forming temperature of the aluminum oxide layer. In this process, the selection of the oxidizing condition for the surface of the layer immediately below the aluminum oxide layer enables the control of the oriented texture coefficient of the aluminum oxide. The oriented texture coefficient also can be controlled by changing the thickness of the aluminum oxide layer under the same oxidizing condition.
In addition to the above-described specification on the constitution of the individual layers, the lifetime of the tool can be further extended when the coating is polished to further decrease the surface roughness so that the surface roughness xe2x80x9cCmaxxe2x80x9d of the outermost surface of the coating satisfies equation 5. Even when this surface improvement is applied only to the cutting edge, the lifetime of the tool can be further extended.
Cmax|Bmax less than 0.5xe2x80x83xe2x80x83equation 5.
It is desirable that as the base material the hard metal be made of a cemented carbide that comprises (a) a hard phase comprising tungsten carbide as the main constituent and at least one member selected from the group consisting of the carbides, nitrides, and carbonitrides of the metals belonging to the IVa, V a, and VIa groups in the periodic table and (b) a binder phase comprising at least one member of the metals belonging to the iron group.
In addition to the foregoing cemented carbide, a thermet alloy that comprises (a) a hard phase comprising titanium carbonitride as the main constituent and at least one member selected from the group consisting of (a1) the carbides, nitrides, and carbonitrides of the metals belonging to the IVa, Va, and VIa groups in the periodic table and (a2) the solid solutions of these and (b) a binder phase comprising at least one member of the metals belonging to the iron group may be used as the hard metal without any problem.
These hard metals can perform a property in which the resistance to wear and the resistance to chipping are particularly well balanced, thus extending the lifetime of the tool.
In the case that the hard metal having tungsten carbide as the main constituent is used as the base material, when the hard metal has at the surface region a layer in which the hard phase except tungsten carbide is reduced or removed (a hard-phase-reduced layer) and when the layer is controlled to have a thickness of 50 xcexcm or less, the resistance to chipping can be further increased.
If the thickness of the hard-phase-reduced layer exceeds 50 xcexcm, the surface region of the base material tends to deform plastically or elastically during the cutting work. The hard-phase-reduced layer can be formed either by the well-known method using a nitrogen-containing hard-phase material or by another method in which a nitrogen-added atmosphere is provided for the temperature-rising period in the sintering process. After the appearance of a liquid phase in the hard phase, the atmosphere is changed to a denitrified and decarburized atmosphere.